Planar electromagnetic transducers comprised of a combination of a permanent magnet (s) and a vibratory diaphragm are known.
An electromagnetic transducer of this type normally includes a permanent magnet assembly, a vibratory diaphragm disposed in opposing relation to the permanent magnet assembly, and a support member for fixing the vibratory diaphragm to the permanent magnet assembly at a peripheral region thereof.
A permanent magnet assembly used in the conventional electromagnetic transducers of this type has a plurality of elongated permanent magnets each having two opposite poles (vertical magnetizing of the assembly) on the surface of both sides thereof, which magnets are arranged in parallel relation such that N-poles and S-poles appear alternately and fixedly jointed together by a non-magnetic component member. The vibratory diaphragm is a thin resin film, on a surface of or within which a coil comprised of a serpentine conductor pattern is formed. The vibratory diaphragm is combined with the permanent magnet assembly such that a linear portion of the conductor pattern will be located right on the central region between the elongated permanent magnets which are arranged in parallel relation. In actual practice, the vibratory diaphragm is fixed to the permanent magnet assembly at a peripheral region thereof through a spacer (s).
A magnetic line of force runs between magnetic poles of two adjacent elongated permanent magnets, and a magnetic field is developed in such a manner as to transverse the linear portion of the conductor pattern of the vibratory diaphragm. When the coil of the vibratory diaphragm is energized, an electromagnetic force is generated in accordance with Fleming's left-hand rule and the vibratory diaphragm is displaced in a thickness-wise direction thereof. According to this rule, vibrations corresponding to the drive current to the coil are generated to create an acoustic wave. This acoustic wave passes through the elongated permanent magnets so as to be radiated outside.
In the conventional permanent magnet assembly, it is desirable to arrange the elongated permanent magnets as densely as possible in order to enhance the efficiency. However, in case the permanent magnet to be used is a sintered magnet (ferrite magnet), for example, the more the effort to form the permanent magnet into an elongated design is increased, the more difficulty is encountered to form it with a precise accuracy (deformation such as warp tends to occur during sintering). Also, it becomes increasingly difficult to provide a sufficient mechanical strength. Moreover, since a large magnitude of magnetic force interacts between the elongated permanent magnets, an extremely difficult work is accompanied in bringing the elongated permanent magnets closer to each other for an accurate assembly. There is another problem in that since the individual elongated permanent magnets are held in their separated states, magnetic poles appear not only on the thickness-wise both sides thereof but also edge portions between the upper and lower surfaces and the side surfaces and a part of the side surfaces, and a magnetic flux jumps laterally between the approaching elongated permanent magnets, with the result that the number of the magnetic lines of force interlinked with the coil of the vibratory diaphragm (that is, a linear portion of the conductor pattern) is undesirably reduced and the driving efficiency is degraded.
As a consequence, the large number of elongated permanent magnets must be mutually arranged at large spaces. For this reason, it becomes necessary to form a large space between the permanent magnet assembly and the vibratory diaphragm. This again degrades the transforming efficiency and increases the thickness of the electromagnetic transducer as a whole.
In the conventional technique, since there is employed a construction in which the vibratory diaphragm is firmly pressed at its peripheral region with a spacer(s), a support point (fulcrum) is created at the peripheral region. As a consequence, somewhat awkward vibrations are generated. This makes it difficult to reproduce high fidelity sounds with respect to the drive current. Moreover, a large amplitude is difficult to obtain.